You are being optimistic. I don't know how many companies have this train first policy. My doesn't.
My personal experience in this regard is with smaller tech and major telco firms (SBC, AT&T, Bell Canada) when I worked for/with them. I've heard, but do not have personal experience, that the Canadian banks generally hold this policy for their travelling staff as well. Where do you work and who books your travel (yourself, or a corporate travel agent)?
I can tell you that getting stuck in traffic and missing a meeting you flew into when there was another option would have been a firing offence for SBC 10 years ago when I was with their subsidiary.
What percentage of business travelers stay right beside Union station?
A majority of business tourists working in the financial district stay within 1km of Union Station. 1km gets you to the Hilton and Sheraton on Queen; covers about 10,000 rooms. The Eaton Chelsea is a major outlier (1,500 rooms). Four Seasons attracts more tourist than business traveller.
If not, that involves another cab ride. Does that make sense to you?
Nothing captures 100% of the market. GO Rex, the main future competition to UPX, is actually the biggest problem UPX will have. Although that airport ticket premium won't disappear (see Sydney where is costs $5 AUD less to go one station further than the airport and walk back).
Georgetown GO Rex will carry the price sensitive passengers who don't mind a `coach class` feeling and service. Don't forget this corridor has another 5 years of construction ahead of it; this really is just the beginning.
UPX is for and will attract the business traveller who is in Toronto for under 24 hours (most of them). That extra 40 minutes of travel time can significantly impact their stay, particularly for a day trip (9am landing, 7pm take-off).
Those who are price sensitive will continue take the bus/subway.
This is a bad argument because extremely few tourists take the bus today when visiting Toronto. Which means their either very few are price sensitive or it's a huge pain in the ass (in reality, it's both). The TTC has about a 1% share of the total airport transportation market and a huge chunk of that is airport/airline employees.
Time sensitive tourists (business tourists mostly) will take UPX. Price sensitive tourists will take the Georgetown GO Rex service.
FYI, the Airport Express is stopping service this fall citing falling passengers loads due in part to increased congestion/travel times. Clearly travel time is a major concern pushing people to take a cab despite the bus being half the price. We will see if UPX is fast enough to attract people.